1. Field of the Invention
The present invention relates to a method for real-time control of the fabrication of a thin-film structure by ellipsometric measurement.
2. Description of the Related Art
Ellipsometry is a non-destructive measurement technique for optically characterizing a specimen having a specular or quasi-specular surface.
Ellipsometry can be used in situ and therefore makes it possible to study the mechanisms involved in the growth of thin layers and in the formation of interfaces and to control the process for fabricating these layers and interfaces. Ellipsometry is, for example, used to study and control the fabrication of semiconductor materials and components.
Ellipsometric measurements may be carried out at a fixed wavelength or at several wavelengths (spectroscopic ellipsometry). Depending on the wavelength range of the optical components used (source, detector, etc.), it is possible to obtain different properties of the layers and of the materials or to explore different materials.
In the ultraviolet and visible range, the depth of penetration of the radiation is quite small. This constitutes favorable conditions for the study of surfaces and interfaces, and for real-time control. But this does not always allow volume properties of the layers and the materials to be obtained, properties which then have to be determined by measurements in the near-infrared range.
The far infrared is well suited to vibrational absorption measurements (chemical bonds).
In order to make ellipsometric measurements, the surface of a specimen is illuminated with a light beam and the state of polarization of an incident beam i is compared with that of the reflected beam r or transmitted beam. The polarization vector E is generally represented by its projections Es and Ep perpendicular and parallel to the plane of incidence, respectively. The projections Ep and Es are complex quantities.
In the field of ellipsometry, the ratio ρ=(Ep/Es)r/(Ep/Es)i, signifying modifications in the state of polarization which are produced by the surface studied, is generally represented in the form:ρ=tan Ψ.exp(iΔ)=(Ep/Es)r/(Ep/Es)i
The two angles Ψ and Δ describing the change in polarization are thus combined in the complex quantity ρ.
The angles Ψ and Δ, and therefore ρ, depend on the properties of the specimen as well as on the angle of incidence of the beam and the measurement wavelength. The expression for Ψ and Δ or for ρ, as a function of these parameters, is given by the Fresnel equations quoted, for example, by D. Charlot and A. Maruani in Appl. Opt. 24, 3368, 1985.
In phase-modulated ellipsometry, an incident ray has its polarization modulated by a phase difference generated between two specific axes of a phase modulator. The phase shift δ(t) typically changes with time t in a periodic angular frequency ω law, δ(t) being proportional to the first order to sin(ωt).
In phase-modulated ellipsometry, the intensity of the light flux reflected by a specimen is used to deduce, in a known manner, the values of Ψ and Δ.
Ellipsometry, and more particularly phase-modulated spectroscopic ellipsometry (PMSE), is a powerful technique for measuring, in real time, the growth of layers on a substrate. This technique has the advantage of not disturbing the process being carried out. Moreover, it is very sensitive to physical parameters of the specimen measured, such as the thickness d of the layer and the refractive index n. Furthermore, it allows rapid measurements (Bernard Drevillon, “Progress in crystal growth on characterisation of material”, vol. 27, 1998, p. 1-87).
According to a known method, the angles Ψ and Δ, or ρ, are calculated from amplitude measurements. These quantities Ψ and Δ depend on physical parameters of the specimen measured, such as the index n and the thickness d of the surface layer. In the case of transparent materials, these parameters may thus be calculated from Ψ and Δ by direct inversion of the Fresnel equations. This inversion must in general be carried out iteratively.
The application of phase-modulated spectroscopic ellipsometry to in situ growth control is, for example, described in the document “High-speed spectral ellipsometry for in situ diagnostics and process control”, Duncan et al., J. Vac. Sci. Technol. B., 12(4), 1994.
It is in fact often difficult to deposit a structure consisting of several layers of different thickness and refractive index, for example by PECVD (Plasma Enhanced Chemical Vapor Deposition) in order to obtain the necessary accuracy (about 2%) required for the production of optical filters for example. It is insufficient to choose the deposition times for each layer from the growth rates of these layers measured during previous experiments. Real-time control, with a feedback loop on the deposition parameters therefore becomes indispensable.
Various approaches have already been followed in order to improve these ellipsometric measurement methods and to apply them to such industrial processes.
In particular, patent FR-2,731,074 proposes to estimate, during a process for fabricating layers, the physical parameters of the said layers from measurements made by ellipsometry and to approximate them using the method of adjustment by the method of least squares of theoretical values taken as reference.
More particularly, these methods generally require intermediate calculations to be carried out on the basis of raw measurements (such as interferential calculations taking into account the totality of the layer deposited), thereby considerably slowing down their implementation.
Another method has also been proposed (M. Kildemo, P. Bulkin, S. Deniau and B. Drevillon, Appl. Phys. Lett. 68, 1996, p. 3395). This consists in measuring, in real time, in the (Is,Ic) plane, where Is and Ic are known functions which will be explained later, the distance between the measured point and the theoretical point corresponding to the end of each sublayer.
This method requires a very precise knowledge of each path end. It therefore has the drawback of being very sensitive to systematic errors, especially in the applications involving multiple wavelengths. These systematic errors (such as those due to ellipsometry calibration errors) will tend to artificially make the experimental path depart from the theoretical path. This may considerably reduce the precision of the control.
It also requires precise optical knowledge about the substrate, this often being difficult in the case of heterogeneous materials or materials which are not very absorbent, such as the standard glasses.
These various methods therefore each have drawbacks—they are either slow or lack precision.